The structure of 2-keto-3deoxy-6-phosphogluconate (KDPG) aldolase will be extended to 2.25 A resolutio using density modification methods and will then be refined employing restrained LS procedures. The refined structure will serve as the basis for interpretation of all subsequent studies probing various aspects of the aldolytic cleavage mechanism. The ES complex will be determined as the binary and ternary ketimine catalytic intermediate (Py-KDPG aldolas, KDPG-KDPG aldolase, respectively) from tetragonal crystals. The conformational change between binary and ternary will be delineated anc correlated to the observed 100-fold increase in reaction rate. The mode of pH activation of Lys 144 will be uncovered from the refined pH 4.2 and 5.3 conformer structures of KDPG aldolase. Enzymic stereo-specificity will be revealed with the structure refinement of the KDPGa1-KDPG aldolase complex, and the phosphate binding site will be confirmed with the produce D-glyceraldehyde-3-P04 binary enzyme complex. A comprehensive three-dimesional mechanism of aldoytic cleavage will emerge from all the foregoing. The structure determination of the central subunit of the multinezyme complex of transcarboxylase will reveal the folding of the 6 subunits, located 12 CoA ester sites in the folded and assembled subunit structure and will identify the nature of the subunit interactions. In addition, the structure will reveal the differences in the opposite faces which lead to an asymmetrical dissociation of the intact multienzyme complex or will establish cooperativity if none exist. These results will also be of immense significance in interpreting the mechanism of catalysis of other biotin enzymes. The refined structure of KDPG aldolase will also be the fires of an 8-fold Alpha/Beta barrel structure at high resolution. At present, there are 7 other functionally very different enzymes with the same peptide folding. KDPG aldolase will serve as the model for the lower resolution and/or unrefined structures. Since the catalytic site of some of these enzymes also occurs in the f-F loop even though a barrel-orientation preference is not shown, comparison of active site regions could reveal evolutionary trends.